Small Plasma Focus Device Research Articles

On the failure of neutron yield scaling in the dense plasma focus

The observed scaling of neutron yield in the dense plasma focus (DPF) as the fourth power of the current in the plasma was the principal driver of the growth of DPF research in its early days. Subsequent discovery of failure of this scaling law was also the principal reason for its abandonment by major laboratories. Attempts to understand this failure of scaling have so far been inconclusive. This Letter looks at this failure in the context of the recently introduced the Generalized Plasma Focus problem and suggests a possible reason that can be experimentally examined using small plasma focus devices. This involves restrictions placed on the drive parameter by conservation laws for mass, momentum, and energy. A suggested empirical workaround to the problem of neutron yield scaling failure could also be configured as a method for increasing the pressure range for neutron emission in small DPF devices.

The cathode array effects on features and reproducibility of emitted hard x-rays, and anode erosion in a small plasma focus device

Using a small (450 J) Mather-type plasma focus device, this research investigates the effects of cathode rod removal from the cathode array in two steps on hard x-ray emission, both in total and specifically from the plasmoid. The effect on anode rod erosion is also measured. Six hard x-ray detectors were placed around the chamber to measure the emitted hard x-rays. By reducing the number of cathode rods, some features of these hard x-rays such as peak intensity and pulse yield decreased and were deteriorated. Besides, the capability of the device for producing and reproducing the hard x-rays with roughly similar and repeatable characteristics diminished. Moreover, the anode erosion took place during the breakdown phase and by using the detectors around the chamber, similar features were obtained both for the total and the plasmoid hard x-rays, but with different scales. The cathode rod removal could violate the symmetry and uniform formation of the current sheath layer during the breakdown phase and could weaken and change the direction of the Lorentz force vector acting on the current sheath layer during the axial phase. It also causes discharge delay and decreases the axial and radial velocities of the current sheath layer which would result in a weak pinch effect and reduced number of energetic runaway electrons inside the plasma column during the pinch phase. These electrons, by colliding the anode surface, are mainly responsible for producing the great portion of the emitted hard x-ray. On the contrary and based on a different mechanism, the cathode rod removal enhances the number of the runaway electrons during the breakdown phase which increases the anode erosion and enhances the impurities that eventually leads to a weak emission of low intensity hard x-rays.

Stable current sheath dynamics caused by preionization source in a small plasma focus device using He as an operating gas

ABSTRACTPinching of the current sheath in the plasma focus (PF) device is an important phase to transfer the stored energy to the high density short lived plasma for the production of a group of useful radiations. The current sheath formation during the initial breakdown of the gas is a critical process in the PF operation. In this regard, the understanding of the current sheath dynamics is of prime importance. In this paper, we present the effects of alpha particle induced preionizaion by U238 in 3.3 kJ He operated PF device on the dynamics of current sheath, focus amplitude and axial rundown velocity by employing Rogowski coil and High voltage probe as main diagnostic tools. The formation of parabolic axisymmetric sheath is confirmed by Rogowski coil and high voltage probe signals which also reveals the variation in focusing time as well as amplitude during collapse phase by using the preionization source. Time resolved X-ray diagnostics are also carried out using PIN diodes covered with Al and Mg filters of 12.5 and 25 µm which provide an important information about an early onset of X-ray generation. Time integrated X-ray imaging is also carried out to estimate X-ray production zone in the PF device indicating enlarged and symmetric images with preionization.

Simultaneous Ultra-Fast Imaging and Neutron Emission from a Compact Dense Plasma Focus Fusion Device

Recently, there has been intense interest in small dense plasma focus (DPF) devices for use as pulsed neutron and X-ray sources. Although DPFs have been studied for decades and scaling laws for neutron yield versus system discharge current and energy have been established (Milanese, M. et al., Eur. Phys. J. D 2003, 27, 77–81), there are notable deviations at low energies due to contributions from both thermonuclear and beam-target interactions (Schmidt, A. et al., Phys. Rev. Lett. 2012, 109, 1–4). For low energy DPFs (100 s of Joules), other empirical scaling laws have been found (Bures, B.L. et al., Phys. Plasmas 2012, 112702, 1–9). Although theoretical mechanisms to explain this change have been proposed, the cause of this reduced efficiency is not well understood. A new apparatus with advanced diagnostic capabilities allows us to probe this regime, including variants in which a piston gas is employed. Several complementary diagnostics of the pinch dynamics and resulting X-ray neutron production are employed to understand the underlying mechanisms involved. This apparatus is unique in its employment of a 50 fs laser-based shadowgraphy system that possesses unprecedented spatio-temporal resolution.

Two hump-shaped angular distributions of neutrons and soft X-rays in a small plasma focus device

Angular distributions of soft X-rays (SXRs) and neutrons emitted by a small plasma focus device (PFD) were investigated simultaneously using TLD-100 dosimeters and Geiger-Muller activation counters, respectively. The distributions represented two humps with a small dip at the angular position 0° and reduced from the angles of ± 15° and ± 30° for the neutrons and SXRs, respectively. The maximum yield of 2.98 × 108 neutrons per shot of the device was obtained at 13.5kV and 6.5mbar. A time of flight (TOF) of 75.2ns between the hard X-ray and the neutron peaks corresponds to neutrons with energy of 2.67MeV. A similar behavior was observed between the angular distributions of neutron and soft X-ray emissions.

The effects of the cathode array on emitted hard x-ray from a small plasma focus device

In this study, the effects of the cathode array variations on emitted hard x-rays from a small Mather type plasma focus device (450 J) were investigated. The gradual elimination of the cathode rods inside the cathode array of the device lowered the quality and quantity of the emitted hard x-rays at different pressure values of argon gas. We theorized that the variations of the cathode array were able to change some discharge parameters that could vary the number of the energetic runaway electrons generated during the pinch phase which were responsible for the created features of the emitted hard x-rays. On the other hand, we hypothesized that the removal of the cathode rods could influence the current sheath dynamics during all the phases of a shot including its average axial velocity which was demonstrated by using two axial magnetic probes. We also theorized that cathode rod omission from the cathode array could also increase the initial inductance and the impedance of the system, and the impurities inside the plasma during the pinch phase which could lead to the growth of the instabilities. Moreover, by using the wavelet technique and studying the Mirnov signals, it was shown that the decrease of the cathode rod number increased the plasma magnetic field fluctuations or instabilities (MHD activities) that adversely affected the pinch quality, and reduced the emitted hard x-rays.

Design and construction of a very low energy plasma focus, operating at repetitive discharge mode and pinching evidences

As a first step toward the demonstration of pinching evidence using a very low energy discharge, we have designed and constructed a very small plasma focus device of 3J (PFD-3J) as a repetitive pulsed and portable radiations source. The system is capable of operating in a repetitive discharge rate up to 5 Hz as well as very low energy consumption (84 nF capacitor bank, 48 nH, 8–9 kV, ∼2.5–3.5 J). Several diagnostic tools, a CMOS digital single-lens reflex camera, current and voltage probe, are employed simultaneously to confirm the occurrence of pinching in the device. When the system is operated in discharges at 0.9 torr argon gas pressure and initial charging voltage of 8.4 kV, the focusing/pinching indications are genuinely observed as the appearance of a sharp voltage spike/intense current drop on the electrical discharge traces. The PFD-3J performance at repetitive discharge mode was also verified by adjusting the high voltage pulsed-power supply (HVPPS) at 5 Hz shot frequency. It was observed that the rise time of 10%–90% of the charging voltage, at each single shot trace was 115 ms as expected from the design.

The influence of the cathode array and the pressure variations on the current sheath dynamics of a small plasma focus device in the presence of an axial magnetic probe

In this research, the influence of the cathode array and the pressure variations on the current sheath dynamics of a small plasma focus device (450 J) was investigated. For this purpose, the signals of an axial magnetic probe for two different gases (argon and nitrogen) were studied. The magnetic probe signals showed the slower movement of the current sheath layer when the number of cathode rods decreased. This was related to the increase in the circuit inductance, which caused the longer discharge time of the capacitor bank followed by the creation of runaway electrons. These electrons in turn produced the impurities that led to the appearance of the instabilities inside the plasma. On the other hand, in order to investigate the effect of the cathode array variation on the instabilities produced inside the plasma, the wavelet technique was used. With the aid of frequency analysis, this technique showed the increase in these instabilities, which was due to the non-uniform formation of the current sheath layer during the breakdown phase, and finally, the higher values of the pressure caused the slower movement of the current sheath due to the inverse relation of the current sheath velocity to the square root of the pressure.

Experimental study of the performance of a very small repetitive plasma focus device in different working conditions

SORENA-1 is a very small repetitive Mather-type plasma focus device (20 J) that can operate at frequencies up to 1 Hz. This device has been designed and constructed in the Plasma and Nuclear Fusion Research School of the Nuclear Science and Technology Research Institute of Iran. In this article, the structure of SORENA-1 is described and results of experiments with Ar, Ne, and D2 working gases at several discharge voltages and initial pressures are presented and analyzed.

Deuterium Plasma Focus as a Tool for Testing Materials of Plasma Facing Walls in Thermonuclear Fusion Reactors

This paper presents the first verification of the fast ion beam (FIB) and fast plasma stream (FPS) properties computed using the Lee code. Recent estimates of FIB and FPS properties in PF-400J from interferometric images are compared to our computed results. Reasonable agreement is found in the comparison of several quantities. Our computed power flow density (energy flux) and integral damage factor are 2.45 × 1012 Wm−2 (2 times of experimental value) and 1.78 × 108 Wm−2 s0.5 (~almost the same as experimental value) respectively, for target placed at 1.5 cm from the anode top of PF-400J. This verification gives us confidence to proceed to systematic numerical calculations on the PF-400J and similar small plasma focus devices (PF50, NX2, NX3, FMPF-3, INTI PF) to obtain FIB properties (ion beam energy, ion beam flux, ion beam fluence, beam ion number, ion beam current, power flow density, and damage factor). These results confirm that the plasma focus, including small ones, could be useful to study the effects of cumulative pulses on target materials being considered for plasma facing walls in future tokamak or laser-implosion fusion reactors.

Comparison of Plasma Dynamics in Plasma Focus Devices PF1000 and PF400

<p>In this work we have simulated two different sizes of plasma focus devices: PF400, a small sub-kJ plasma focus device in operation at the Atomic Energy Commission in Chile and PF1000, a 1 MJ device at the International Centre for Dense Magnetised Plasmas (ICDMP) in Warsaw, Poland. Our aim is to compare the plasma dynamics observed in two different sizes of the devices. We compare the experimentally obtained data with the data predicted from our numerical experiments using the Lee model code. It has been observed that the peak current of the total current waveform, pinch current calculated from total current, and neutron yield agree reasonably well with published experimental data, however, slight differences are found in other parameters.</p><p>Journal of Nepal Physical Society Vol.3(1) 2015: 55-59</p>

Angular distribution of ion beam emitted from a 3.5 kJ plasma focus device using different shapes of anodes

The distribution of argon ion beam emitted from a small plasma focus device (PFD) with four different anode tips i.e., cylindrical with a flat top, cylindrical with a hollow top, cone with a flat top, and cone with a hollow top is studied at various working pressures. The angular distribution of ions is significantly reduced at angles higher than ±11° and the maximum ion emission is between 0° and 11°. The maximum ion flux of about 5.57×1012 ions/steradian is obtained with cylindrical-flat anode tip that increases to 9.82×1012 ions/steradian per shot for cone-flat anode tip.

High-Voltage Pulsed-Power Supply Operating at Repetitive Discharge Mode for Driving Very Small Plasma Focus Devices

The technology of switched-mode power supplies (SMPSs) has been employed to design and build a much simpler and portable high-voltage pulsed-power supply (HVPPS) as an efficient pulse drive unit for very small plasma focus devices (PFDs). The HVPPS is designed in such a way that it is able to charge a low-energy capacitor bank (84 nF, 7 kV, 2.1 J), from 4 to 7 kV at a repetitive discharge mode of 5 Hz. All control processes are carried out via a laptop through a graphical interface, written in C# language. The experimental runs were carried out in argon at 4-mbar gas pressure and the 7-kV maximum charging voltage level of the HVPPS. The results of diagnostics at a repetitive discharge mode of 5 Hz successfully confirmed the accurate performance of the SMPS and the spark gap switch for driving a very small PFD. The rise time of 10%–90% of the charging voltage, at each single-shot trace was obtained to be 110 ms. This consequence demonstrated that the control and sampling circuits were being implemented as well. After discharging, the pinching evidence was genuinely observed in the capacitor bank voltage trace. The quarter period of the discharge traces was observed to be 100 ns, which leads to a total system inductance of about 48 nH. The measured inductance was low enough to be compatible with successful performance of the very low energy PFD, leading to fast and high magnitude discharge pulses to assist the pinching evidence. This HVPPS could emerge as one of the very low cost, compact, light weight, and highly efficient power supplies used for driving the very small PFDs so far.

Neutron Yield Scaling With Inductance in Plasma Focus

Neutron yield is enhanced by reduction of the short-circuit plasma focus inductance $L_{\mathrm { {0}}}$ , which in turn increases the pinch current $I_{\mathrm { {pinch}}}$ . Numerical experiments are carried out to optimize the neutron production using the Lee model on small plasma focus devices (several kilojoules), such as UNU/ICTP PFF, NX2, and LDiana. The maximum neutron yield is obtained for each $L_{\mathrm { {0}}}$ with a combination of gas pressure as well as anode length and radius ( $Y_{n}$ versus $z_{\mathrm { {0}}}$ , $p_{\mathrm { {0}}}$ and $a$ at fixed $L_{\mathrm { {0}}})$ . The numerical experiments confirm the limitation effect of $I_{\mathrm { {pinch}}}$ , and suggest an optimum $L_{\mathrm { {0}}}$ in the range of 10–20 nH. Finally, the neutron yield is scaled with system inductance.

Influence of the Anode Length on the Neutron Emission of a 50 J Plasma Focus: Modeling and Experiment

A comprehensive set of electric data measured in a small plasma focus (PF) device of 50 J correlated with the corresponding neutron emissions is taken as the base for developing a semiempirical model of the current sheet dynamics and the neutron yield. The model is able to explain the dependence of the neutron yield with the pressure and anode length with good accuracy, and suggests a physical interpretation of the drive parameter commonly used in PF design.

Simulation of electrical discharge in a 3.6 Joule miniature plasma focus device using SIMULINK

A novel technique has been developed and studied in this paper to simulate the electrical discharge circuit of a 3.6 J miniature plasma focus device (PFD) and investigate the effect of inductance variation on voltage spike and current dip. The technique is based on a correlation between the electrical discharge circuit and plasma dynamics in a very small PFD that operates at the energy of 3.6 J. The simulation inputs include the charging voltage, capacitor bank capacitance, current limiter resistance, by-pass resistance as well as the time-dependent inductance and resistance of the plasma sheath which are calculated by assuming the plasma dynamics as transit times in going from one phase to the next. The variations of the most important elements in the circuit (i.e. the constant and breakdown inductances) and their effects on the current dip are studied in PFDs with low and high constant inductance. The model demonstrated for achieving a good pinch in the PFD, although the total inductance of the system should be low; however there is always an optimum inductance which causes an appropriate pinch. Furthermore, the electrical power produced by the pulsed power supply, the mechanical energy as well as the magnetic energy which are transferred into the plasma tube were obtained from simulation. The graph of electrical power demonstrated a high instantaneous increment in the power transferred into the plasma as one of the greatest advantages of the pulsed power supply. The simulation was performed using software tools within the MATLAB/SIMULINK simulation environment.

Measurements of ions emission using ToF method and CR39 SSNTD in a Small Plasma Focus device of Hundreds of Joules

Ion beam emission in plasma focus (PF) discharges was originally investigated to explain the strong forward anisotropy observed in the neutron emission when D2 is used as filling gas in the PF devices. Several properties of emitted deuteron beam, including its angular distribution and energy spectra in PF devices operating at energies from 1kJ to 1MJ, have been measured. At present there is a growing interest in the development of very small PF devices operating under 1kJ. As part of the very low energy (< 1kJ) PF devices physics characterization program carried out at the Chilean Nuclear Energy Commission, the charged particle emission is being studied when hydrogen (H2) and mixture (H2+ %Ar) is used as filling gas in the PF device. The experiments have been performed in a plasma focus device of 400 joules (PF-400J). In order to estimate the ion beam energy spectrum and its ionization degree, by means of the time of flight method, a graphite collector system operating in the bias ion collector mode was constructed. On the other hand, measurements of the energy and flux of the ions have been corroborated using CR39 SSNTD. Preliminary results mainly show the presence of hydrogen ions with an average energy of 40keV. Also, in some cases, copper (from anode) and aluminum (from insulator) ions with energies lower than 1keV have been observed.

Assessment of image quality in x-ray radiography imaging using a small plasma focus device

This paper offers a comprehensive investigation of image quality parameters for a small plasma focus as a pulsed hard x-ray source for radiography applications. A set of images were captured from some metal objects and electronic circuits using a low energy plasma focus at different voltages of capacitor bank and different pressures of argon gas. The x-ray source focal spot of this device was obtained to be about 0.6mm using the penumbra imaging method. The image quality was studied by several parameters such as image contrast, line spread function (LSF) and modulation transfer function (MTF). Results showed that the contrast changes by variations in gas pressure. The best contrast was obtained at a pressure of 0.5mbar and 3.75kJ stored energy. The results of x-ray dose from the device showed that about 0.6mGy is sufficient to obtain acceptable images on the film. The measurements of LSF and MTF parameters were carried out by means of a thin stainless steel wire 0.8mm in diameter and the cut-off frequency was obtained to be about 1.5cycles/mm.

Design and construction of a very small repetitive plasma focus device

In this paper the design and construction of a very Small Plasma Focus Device (20J) that can operate at repetition rate about 1 Hz are described. In the first experiments with it using Ar as working gas, high negative spikes in current derivative signals have seen, and the proper range of the operation using the Ar is identified.

Neutron Production From a Small Modified Plasma Focus Device

In this paper, the results are presented of the study of the pinch evolution in the modified small plasma-focus device at the current peak of 200 kA and with a deuterium filling. The disk conducting the cathode was placed 1 cm in front of the inner electrode (anode). The neutron yield reached 106-108 per shot. The evolution of the plasma was imaged with four pinhole XUV frames and two schlieren pictures. The neutron signals were registered with three scintillation detectors. The figures and signals were enabled to describe the pinch phase at the moment of hard X-ray and neutron production with a 2-ns temporal uncertainty. The results were similar to the results obtained on the PF-1000 at the current of 2 MA. The first neutron pulse is produced at the stagnation, and the dominant neutron production correlates with the disintegration of the dense plasma column. Additionally, the energies of fast neutrons and deuterons producing neutrons have similar values and distributions as in PF-1000.